Insurers are pressing ahead with a final marketing push to bring as many young, healthy customers as possible onto their rolls and buttress a recent surge in health-law enrollments.

The flood of late sign-ups that helped boost the marketplace total to six million enrollees, a key milestone for the Obama administration, has also brought some insurers an uptick among younger people. But it isn't clear if the trend is broad enough to balance out an earlier skew toward older enrollees, who are more likely to have costly ailments.

"We are seeing our average age come down every week, so it's clear that younger people are starting to come into the pool," said
Wayne DeVeydt,
the chief financial officer of
WellPoint Inc.
"What isn't clear yet, though, is, did it come down enough." WellPoint has said the demographics of its sign-ups have generally matched its projections.

Highmark Inc., a major health plan based in Pittsburgh, said in recent weeks that it had seen a "marked increased" in enrollees younger than 34. Blue Cross & Blue Shield of Rhode Island said its fastest-rising segment in March has been people ages 22 to 40.

Young people wait in line at a health-insurance enrollment event in Cudahy, Calif., on Thursday.
Reuters

Florida Blue Senior Vice President
Jon Urbanek
said "younger people are signing up," but the insurer doesn't know if that will move the dial in a customer pool that had been "skewing older than we anticipated."

Medical Mutual of Ohio said its enrollment through the health-care marketplace has gotten younger each week, and the average age is now a decade below where it was when enrollment kicked off in October. But, the company said, the average is still eight years older than the company projected when it set prices for 2014.

Insurance officials also caution that age doesn't always indicate health status—younger people may have serious, expensive conditions, while some older people rarely need medical services. Age is a "pretty good predictor," said Tom Snook, an actuary with Milliman Inc. who works with insurers offering plans on public exchanges, but "it's not even close to a perfect measure."

So far, insurance carriers have limited insight into the health needs of their new enrollees. Under the law, insurers can't deny coverage or charge higher prices based on health status, and enrollees need to provide only limited information, including age, when they sign up through the marketplaces. Enrollees must start the process of choosing a plan by March 31 to avoid penalties. The Obama administration has extended a grace period to complete enrollment even after the deadline.

As the deadline looms, it isn't clear just how broad the uptick in youth sign-ups has been. HealthMarkets Inc., a health-insurance agency, said its age balance for enrollees hasn't changed in recent weeks. EHealth Inc., which tracks the average age of individual purchasers of nonmarketplace plans through its site, shows it flat in recent weeks. GoHealth LLC, another major health-insurance site, said it had seen an increase in young customers.

To prod a big final wave, insurers, exchanges, health-care providers and others are amping up their enrollment push with a blitz of countdown ads and events. Blue Shield of California is sponsoring events across the state, including sign-ups this weekend at all 42 stores of a Southern California grocery chain with many Hispanic customers. Land of Lincoln Mutual Health Insurance Co. in Illinois parked a tractor-trailer emblazoned with its orange logo outside a hospital sign-up event on Friday.

Health plans are particularly hoping to reach "young invincibles" like Trevor Dawes, a 23-year-old apprentice plumber from the Queens borough of New York City who said he is planning to shop for a plan through New York's insurance marketplace this weekend, ahead of the deadline. He learned recently from a video on Facebook that he could face penalties for going without insurance, which he hasn't had for about a year. "I'm healthy, and I didn't even know it was important," he said.

Arches Health Plan in Utah plans to keep up its push past March 31 to capture late finishers. "We're going right up to the bell," said Shaun Greene, the company's chief operating officer.

Independence Blue Cross, which sponsored a contest to create short digital films about health insurance, is turning the lobby of its downtown Philadelphia headquarters into an enrollment site this weekend and Monday, with radio stations including a hip-hop one broadcasting live from the event. The insurer said the average age of its enrollees has dropped by 1.5 years since January.

Arches will sport sign-up tables at three Utah Jazz basketball games in the first weeks of April. Mr. Greene has enlisted his 17-year-old son and some of his son's football teammates to blanket cars in Wal-Mart parking lots Friday with fliers bearing slogans such as "Peace of Mind Is Priceless."